Pest-resistant Bt corn has helped suppress the European corn borer because the plant provides non-Bt corn in the vicinity protection from the insect, researchers say.

Planting genetically modified, pest-resistant corn can provide a halo effect — offering protection from insects to nearby corn plants that have not been engineered to kill bugs, scientists said Thursday.

Since its introduction in 1996, Bt corn — so called because it has been engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis — has effectively suppressed the European corn borer, a widespread pest in the U.S., according to new research published in the journal Science.

This suppression has saved farmers in Minnesota, Illinois, Wisconsin, Iowa and Nebraska $6.9 billion in reduced yield losses over 14 years, the authors said. And more than half of that economic benefit, they showed, was generated in non-Bt corn acres — fields where farmers enjoyed the insect-fighting benefits from Bt corn because there were fewer borers around to feast on the fields.

"That was surprising to see," said University of Minnesota entomologist William Hutchison, lead author of the Science paper. "Only about 35% of corn acres are non-Bt, but two-thirds of the economic benefit occurs there."

The research provides the best long-term, widespread evidence so far of the effectiveness of Bt corn, said University of Arizona entomologist Bruce Tabashnik, who wasn't involved in the research.

"In areas of pest management and ecology, it's easy to make predictions. It's harder to test, and even harder to validate on such a large scale," said Tabashnik, who wrote an editorial in Science accompanying the paper. "Even a 'huge' field experiment is hundreds of acres. This was tens of millions of acres."

The European corn borer has been a problem in the U.S. since its arrival in 1917. Adult moths lay eggs on the undersides of corn leaves. Larvae hatch and eat the leaves and, as they grow, bore into stalks and ears of corn. The pest reduces corn yields by disrupting nutrient and moisture flow through the stalk, as well as by directly damaging the kernels as it feeds.

It causes about $1 billion in estimated losses each year, Iowa State University researchers have reported.

Universities, state governments and corporations have tracked the European corn borer for decades. Hutchison and his team sifted through more than 50 years of reports on the pest's larvae and moths — "data that had been sitting in file cabinets," Hutchison said — to measure changes in the insect's population sizes over time.

Then they compared the population data against annual U.S. Department of Agriculture statistics — on corn yield, price and areas planted — to estimate Bt corn's economic benefit.

This research, using state, federal and university funding, marks the first time economists have estimated the benefits of Bt corn for non-Bt crops, said study coauthor Paul Mitchell, an agricultural economist at the University of Wisconsin in Madison.

"The focus has always been on the Bt corn and the farmer growing it," he said. "But if [Bt corn] really suppresses the pest, it's a bigger story."

The findings could affect how farmers and policymakers manage Bt crops in the future, Mitchell said. Proving how well Bt corn works may, paradoxically, encourage farmers to plant less, not more, of the genetically modified crop in the future, in hopes that they could reap the benefits of Bt protection without having to pay for as many of the more expensive seeds.

That could be a good thing, Mitchell said, because it could help prevent the corn borer from developing resistance to Bt toxins.

Until 2008, farmers planting Bt corn were required by the Environmental Protection Agency to reserve at least 20% of their fields for non-Bt plants. Scientists hypothesized — correctly, it now appears — that such non-Bt "refuges" would help stave off Bt resistance among corn borers.

But some farmers reportedly resisted the rules, planting more than 80% of their crops with Bt corn to make sure they didn't lose a portion of the crop to the borer. The new research suggests that farmers have nothing to fear from planting non-Bt refuges alongside Bt fields. They may make more money by doing so.

In fact, farmers may stand to gain by planting even more non-Bt corn than the EPA requires, scientists said. Hutchison said that if he were a farmer, he thought he might use a 60% Bt, 40% non-Bt mix, or even a 50-50 combination.

But Margaret Mellon, director of the food and environment program at the Union of Concerned Scientists, which advises a cautious approach to genetically modified crops, wondered whether the savings attributed to Bt corn was enough to merit fanfare. By her estimates, the savings ran to only about 3% of the total value of the corn crop in the five states.